The present invention relates generally to a wireless communication system, and more particularly to a wireless communication system with transmit diversity designs.
Antenna diversity is a technique in which an information-carrying signal is transmitted via a number of propagation paths in a wireless communication system. The antenna diversity technique can be further categorized into receive diversity, transmit diversity, and multiple-input-multiple-output (MIMO) schemes, depending on the number of antennas used at the signal receiving end or the signal transmitting end. The receive diversity refers to a system where the signal receiving end employs multiple antennas to receive multi-path signals from the transmitting end implemented with one antenna. The transmit diversity refers to a system where the signal transmitting end employs multiple antennas to transmit signals via multiple paths to the receiving end implemented one antenna. The MIMO scheme refers to a system where both the transmitting and receiving ends employ multiple antennas to transmit or receive signals via multiple paths.
A typical wireless communication system usually employs the receive diversity technique to improve the quality of the signal reception. Conventionally, the receive diversity technique can be implemented in a base station with many different approaches. The most common approach is placing antennas of similar polarization at the same elevation with the same azimuth by a predetermined space. These separately placed antennas receive signals from multiple paths, and therefore reduce the probability that all of the antennas are blocked from receiving signals as opposed to a system with only one receive antenna. Another common approach is placing two antennas orthogonal in polarization at the same location for signal reception. When either of these approaches is properly implemented, improvement in uplink signal reception is noticeable.
Although receive diversity is commonly implemented at base stations in wireless communication systems, transmit diversity is not as commonly used. A base station usually has sufficient real estate for accommodating multiple antennas, whereas, for cost considerations, a mobile unit often has only one antenna supported by a single amplifier. As a result, a base station implemented with receive diversity needs to switch to a single antenna mode during transmission in order to accommodate the single-antenna mobile units.
FIG. 1 illustrates a conventional wireless communication system 100 comprised of at least one base station 102 and at least one mobile unit 104. The base station 102 includes multiple antennas 106 and 108 coupled to a combiner 114, which is further coupled to power amplifiers 110 and 112. When the base station 102 is in a signal reception mode, its antennas 106 and 108 receive uplink signals transmitted from an antenna 116 of the mobile unit 104 through multiple propagation paths. The combiner 114 then passes the signals received at the antennas 106 and 108 to the power amplifiers 110 and 112 for signal amplification for further processing. When the base station is in a signal transmission mode, the transmit diversity technique cannot be applied because the mobile unit 104 with only one antenna 116 does not support it. As a result, the combiner 102 would combine outputs of the power amplifiers 110 and 112 into one radio frequency (RF) signal, and select one of the antennas 106 and 108 for transmission.
One drawback of the conventional wireless communication system as shown, for example, in FIG. 1, is that it suffers from poor quality of downlink signal reception due to reasons, such as fading, as it does not utilize the transmit diversity technique. Another drawback of the conventional wireless communication system as shown, for example, in FIG. 1, is that it requires a combiner 114 coupled between the antennas 106, 108 and the power amplifiers 110, 112, in order to support a single antenna transmission mode. As a result, the system design is complicated, and its cost is increased.
What is needed is a wireless communication system with a simplified design that supports both the receive diversity and transmit diversity techniques.